1. Field of the Invention
The present invention relates to binder used for binding electrode material such as carbon and the like, specifically to fluoropolymer-based binder suitable for manufacture of battery electrodes.
2. Description of Related Art
Recently, demands have been rising for small size and suitably portable electrical and electronic devices, such as audio tape recorders, cameras, integrated video tape recorders, personal computers, cellular telephones, and the like. This has created a need for small high performance batteries. Among new types of batteries that have been commercialized are nickel hydrogen, and lithium, in addition to traditional lead batteries and/or nickel-cadmium batteries. In addition, development and commercialization are also in progress on relatively large size electrical energy supply devices such as automotive fuel cells, and electrical double layer capacitors.
Materials for production of electrodes for constructing batteries are normally shaped into the preferred configuration using binders, said materials including manganese dioxide (MnO2), nickel hydroxide [Ni(OH)2], hydrogen occlusion alloys, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), carbon, and graphite. Binders have been based on fluoropolymers, which exhibit excellent chemical resistance and heat resistance as well as binding properties.
For example, Unexamined Japanese Patent Application Publication Kokai S63-236258 discloses the use of an aqueous dispersion of polytetrafluoroethylene (PTFE) for binding positive electrode materials for primary lithium batteries such as MnO2, acetylene black, and graphite. Japanese Patent Application Publication H6-10980 describes examples of binding manganese oxide for an air battery, carbon black, and activated carbon using aqueous PTFE dispersion.
Also, examples are known in which polyvinylidene fluoride (PVDF) is used as a binder. Kokai H6-44964 describes mixing a conductor such as a hydrogen occlusion alloy, or nickel carbonyl powder, for a nickel hydrogen battery with a PVDF solution, fabricating the mixture into sheet form, and using the product as an electrode. In the case of a lithium ion secondary battery, as described in Kokai H4-249860, a positive electrode material comprised of a lithium-containing oxide such as LiCoO2 and graphite, and a carbonaceous material as a negative electrode material, are each mixed an N-methyl-pyrrolidone solution of PVDF to be processed into sheet form; here again PVDF is used as a binder.
PTFE particles exhibit the property whereby their surfaces tend to fibrillate when subjected to low shear force, so that mixing them with another powdery material causes them to fibrillate easily. Although fibril formation improves the binding force with electrode materials, fibrils may entangle with each other and hinder uniform mixing. Therefore, uniform mixing is conventionally promoted by converting PTFE from a powder to an aqueous colloidally dispersed product. However, use of PTFE in the form of aqueous dispersion requires, in the final stage, a step of removing by heating, the large amount of water and surfactants contained therein as stabilizers. In addition, the added surfactant and water sometimes adversely affect battery properties. For example, LiNiO2, which is used as a positive electrode material in secondary lithium ion batteries, readily reacts with water, which makes it difficult in practice to use with an aqueous PTFE dispersion.
Since PVDF is soluble in organic solvents, it can be mixed with electrode materials without using water or surfactant. However, the fact that the PVDF is soluble in organic solvents means that the PVDF is sensitive to swelling in the organic electrolytes commonly used in batteries, such as propylene carbonate, dimethoxyethane, γ-butyrolactone. A swollen electrode can degrade battery performance. PVDF is also said to be somewhat lower in binding strength than is PTFE.
Performance of electrical and electronic devices can be enhanced by improving battery properties such as useful life and improved discharge characteristics. Each time secondary batteries are subjected to charging and discharging, the electrodes undergo expansion and contraction, so that if the strength of the binder is inadequate, active materials may slough off from the electrode, resulting in a gradual decline in battery life.
Binder strength can be increased by increasing the amount of PTFE or PVDF binder used. However, fluoropolymers are insulators, and their presence hinders the flow of electricity in the battery. Therefore, for increased battery performance it is desirable to use less fluoropolymer binder, not more.
It is an object of this invention to provide a binder for electrodes which can be uniformly mixed with the electrode material and which exhibits high binding strength at a low levels of addition.